Mechanisms of vitamin B12 absorption in breast-fed infants

Objectives: The mechanisms of vitamin B-12 absorption in infants are unknown. We investigated whether haptocorrin (HC), a vitamin B-12-binding protein in human milk, facilitates vitamin B-12 absorption during the neonatal period or if it occurs by a process similar to that in adults involving another vitamin B-12-binding protein, intrinsic factor (IF). Methods: To determine whether HC or IF can deliver vitamin B-12 to the enterocyte, binding studies using Caco-2 intestinal cells in culture and purified human milk HC-[Co-57]vitamin B-12 or [I-125]IF-vitamin B-12 were performed. Determination of IF secretion by infant stomach was investigated by a competitive ELISA on fecal extracts from breast-fed infants. Determination of receptors specific for IF-vitamin B-12 or HC-vitamin B-12 in infant intestine was achieved by ligand blot analysis using isolated brush border membrane vesicles (BBMV) from fetal and adult intestine and Caco-2 cells. PCR was performed to identify the IF receptor gene transcript in Caco-2 cells and fetal intestine. Results: Limited binding of both HC and IF to Caco-2 cells was observed; however, HC displayed affinity to low molecular weight proteins in BBMV from fetal intestine and Caco-2 cells while IF showed affinity for a 240 kDa protein in BBMV from fetal intestine and Caco-2 cells. IF receptor gene transcript was identified in fetal intestine and Caco-2 cells. An increase in IF excretion from breast-fed infants throughout early life was observed. Conclusions: An IF-dependent vitamin B-12 absorption mechanism appears to be in place in breast-fed infants. However, IF levels may be too low in early life to participate in vitamin B-12 absorption; therefore, haptocorrin may mediate vitamin B-12 absorption until the absorption function can be taken over by a more mature IF system.